SEOUL (AP) — North Korea threatened Thursday to increase punishment for an American who was sentenced to hard labor for illegally entering the country, citing what it called a hostile U.S. policy toward it.

Aijalon Mahli Gomes, from Boston, was sentenced in April to eight years of hard labor and fined $700,000 for entering the country illegally and for an unspecified “hostile act.”

Thaleia Schlesinger, a spokeswoman for Mr. Gomes‘ family in Boston, said they had not heard the news and did not immediately have a comment.

Communist North Korea has freed three other Americans detained for illegal entry, but ruled out Mr. Gomes‘ release amid tensions over the sinking of a South Korean warship that Seoul and Washington have blamed on the North.

South Korea has asked the U.N. Security Council to censure North Korea over the sinking. The North denies it was responsible and has warned that any moves to punish it at the U.N. could lead to armed conflict and possibly nuclear war.

The U.S. and South Korea have urged North Korea to avoid provocations and vowed to hold it accountable for the sinking of the warship in March, in which 46 South Korean sailors died.

North Korea is examining what harsher measures to take against Mr. Gomes under a wartime law, and would be compelled to consider applying the law if the U.S. persists in its “hostile approach,” the official Korean Central News Agency reported Thursday.

“The U.S. government is requesting the [North] to leniently set him free from a humanitarian stand, but such thing can never happen under the prevailing situation and there remains only the issue of what harsher punishment will be meted out to him,” KCNA said.

The U.S. and North Korea do not have diplomatic relations. The 1950-53 Korean War ended in a cease-fire, not a peace treaty.

An expert on North Korean legal affairs said the threat could be a tactic to head off U.S. sanctions over the ship sinking.

“The North is using Gomes as a negotiating card as it knows that the U.S. will not sit idly by about him,” said Choi Eun-suk, a professor at the Institute for Far Eastern Studies at Kyungnam University in Seoul.

Mr. Choi and South Korea’s Unification Ministry, which handles North Korean affairs, said they had no knowledge of the wartime law mentioned by KCNA.

The report said North Korea considers the current situation “a war phase” and is handling “all relevant issues according to a wartime law,” apparently referring to tensions over the sinking.

Little is known about the conditions under which Mr. Gomes is being held. In late April, he was allowed to speak to his mother by telephone.

His motivation for entering North Korea is also unclear. He had been teaching English in South Korea before being arrested in the North on Jan. 25.

He had attended rallies in Seoul in support of Robert Park, a fellow Christian who deliberately crossed into North Korea from China to call attention to the North’s human rights record.

Mr. Park was expelled from North Korea about 40 days after entering the country last Christmas. American journalists Laura Ling and Euna Lee, who also crossed the border into North Korea, were held for five months before being released last August during a trip to North Korea by former President Bill Clinton.